1. Field of the Invention
The present invention relates to a method for stabilizing a low-concentration standard reference gas, and a low-concentration standard reference gas obtained by such a method.
2. Description of the Related Art
In recent years, a variety of harmful substances are found at a low concentration in the atmosphere. Potential harm to the human health due to a long-time exposure to these substances has become a concern. Thus, it is necessary to monitor the concentration of these substances to see if the concentration exceeds a certain level.
In measurement of a harmful contaminant in the atmosphere, a greater reliability (measuring accuracy) is required as the quantity of the measured substance is smaller. Generally, the measurement is made by an analyzing instrument, and the reliability of the instrument must be maintained by periodically performing an accurate calibration with the use of an appropriate standard reference material. Specifically, a standard reference material containing a known concentration of harmful material is measured by the analyzing instrument which is then calibrated so that the measured concentration coincides with the known concentration of that harmful substance.
When the harmful substance is a gaseous component in the atmosphere, use is made of a standard reference gas containing that harmful substance as a component. In order for the calibration of the analyzing instrument to be made properly, the standard reference gas must be highly stable and accurate in its concentration level for a long time. However, because the standard reference gas is usually charged in a high-pressure gas container before supply, the gas component may react with the inner wall surface of the container and/or adsorbed by and desorbed from the inner wall surface of the container when the gas component is highly reactive and/or adsorptive, which leads to concentration fluctuations. Such concentration fluctuations are more significant as the concentration becomes lower.
Conventionally, for concentration stabilization of a gas component in a standard reference gas, the inner wall surface of a high-pressure gas container is treated by plating, coating or polishing. For example, Japanese Patent Publication 54-134070 discloses a method for stabilizing a standard reference gas by coating the inner wall surface of a high-pressure gas container with a natural or synthetic wax.
Further, in case where a standard reference gas contains a highly reactive gas component, the gas component is first charged in the container at a higher concentration (several times to several hundreds of times higher) than the target concentration, whereby the container is left for a certain period of time for aging to allow its inner wall surface to become sufficiently inactive. Then, an additional amount of the gas component is charged in the container together with a dilution gas to prepare a standard reference gas having a predetermined concentration of the gas component.
However, the above-described measures for stabilizing a standard reference gas cannot be used for a certain gas component. For example, a standard reference gas containing a trace quantity of acrylonitrile has been found to undergo great concentration fluctuations, thus failing to meet the requirements for a standard reference gas.
It is, therefore, an object of the present invention to provide a method for stabilizing a low-concentration standard reference gas which suffers only little concentration variation during long-term storage even if the standard reference gas contains such a gas component as acrylonitrile, and which is capable of exhibiting an accurate concentration of the gas component especially in a low-concentration range.
Another object of the present invention is to provide a low-concentration standard reference gas obtained by such a method.
According to a first aspect of the present invention, there is provided a method for stabilizing a low-concentration standard reference gas comprising the steps of: treating an inner wall surface of a container with high-purity water for causing the inner wall surface to adsorb the high-purity water; and charging the container with the standard reference gas.
With the stabilization method described above, the inner wall surface of the container is pre-treated with high-purity water. Because of this pretreatment, the molecules of high-purity water are adsorbed and held by the inner wall surface of the container in a thermodynamically stable state. Thus, when a standard reference gas containing a trace quantity of gas component is charged later into the container, the molecules of the gas component will not replace the water molecules which have been previously adsorbed, nor will they react with the inner wall surface of the container. As a result, the concentration of the gas component in the standard reference gas is kept stable for a long period of time.
The step for adsorption may comprise evacuating the container to a vacuum of not higher than 13 Pa, and vaporizing under said vacuum 2xcx9c23 mg, preferably 4.75xcx9c23 mg, of high-purity water per 1 dm3 of volumetric capacity of the container. If the amount of water is less than 2 mg, water adsorption cannot cover the entire inner wall surface of the container. The amount of water exceeding 23 mg does not further improve the effectiveness of the adsorption treatment while deteriorating the stability of the gas component concentration.
The purity of water to be used in the method of the present invention should be as high as possible. Generally, satisfactory results maybe obtained when the high-purity water has a specific resistance of not lower than 0.1xc3x97106 xcexa9xc2x7cm, preferably 10xc3x97106 xcexa9xc2x7cm, particularly 16xc3x97106 xcexa9xc2x7cm. It should be noted here that the specific resistance is a measure representing the purity of water. A higher specific resistance indicates a higher purity, containing a smaller amount of impurities such as ions. There is no upper limit for the specific resistance of high-purity water, but in practice the specific resistance is limited to somewhere around 18xc3x97106 xcexa9xc2x7cm due to the ability of a super high-purity water system.
Typically, the standard reference gas may contain at least one gas component selected from a group consisting of acrylonitrile, 1,3-butadiene, formaldehyde, vinyl chloride, dichloromethane, chloroform, 1,2-dichloroethane, benzene, tricholoroethylene, and tetrachloroethylene. Excellent results may be obtained when the standard reference gas contains acrylonitrile, 1,3-butadiene and formaldehyde.
Further, the stabilization method according to the present invention become more remarkable as the concentration of the gas component in the standard reference gas is lower. Specifically, the concentration of the gas component may be preferably in the range of 0.001 to 100 ppm.
The container may be made of an alloy selected from a group consisting of an aluminum alloy and a manganese steel. Further, the inner wall surface of the container may be polished.
According to a second aspect of the present invention, there is provided a combination of a low-concentration standard reference gas and a container containing the standard reference gas, wherein the container having an inner wall surface which has adsorbed high-purity water.
The other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiment given with reference to the accompanying drawings.